Uv-initiated thermally cross-linked acrylate pressure-sensitive adhesive substances

ABSTRACT

Process for UV-initiated thermal crosslinking within a hotmelt process for the preparation of polyacrylate hotmelt adhesives, in which a polymer is produced from an acrylate-based comonomer mixture that includes monomers which carry at least one functional group capable of reacting with a photochemically generated base, with or without the addition of a catalyst, and then coating this polymer onto a backing, irradiating within the coating process or thereafter, and finally subjecting the polymer to thermal treatment, with crosslinking occurring between the polymer and the photochemically generated base. Structural crosslinking of the sheetlike pressure-sensitive adhesive can be produced if irradiation takes place through a mask.

The invention relates to a new UV-initiated thermal crosslinking ofacrylate pressure-sensitive adhesives both by comprehensive and byselective irradiation.

Within the field of pressure-sensitive adhesives (PSAs), ongoingtechnological developments in the coating process mean that there is aprogressive need for new developments. In the industry hotmelt processeswith solvent-free coating technology are of increasing importance in thepreparation of PSAs, since the environmental regulations are becomingever greater and the prices of solvents continue to rise. Consequentlysolvents are to be eliminated as far as possible from the manufacturingoperation for PSA tapes. One rational route would be the introduction ofthe hotmelt technology. The introduction of this technology, however,also imposes very great requirements on the PSA. Acrylate PSAs inparticular are the subject of very intensive investigations aimed atimprovements. For high-end industrial applications preference is givento polyacrylates, on account of their transparency and weatheringstability. As well as these advantages, however, these acrylate PSAsmust also meet stringent requirements in respect of shear strength andbond strength. This profile of requirements is met by polyacrylates ofhigh molecular weight and high polarity, with subsequent, efficientcrosslinking. The drawback of these polar, high-shear-strength PSAs,however, is that they are unsuited to the operation of hotmeltextrusion, since high application temperatures are required and since,moreover, the molecular weight of the polymer is reduced by shearing inthe extruder. This damage significantly lowers the level of adhesiveperformance. The bond strength and the tack are generally low, since theglass transition temperature is relatively high, owing to the polarfractions in the adhesives. The shear strengths in particular ofhotmelt-coated acrylate PSAs drop significantly in comparison to theoriginal solvent-coated PSA. One major reason lies in the quality ofcrosslinking, since acrylate PSAs from solution are generallycrosslinked thermally. Thermal crosslinking possesses a great advantage:low molecular weight constituents in the polymer are crosslinked as welland so thermally crosslinked PSAs exhibit a higher level of cohesion.

Acrylate hotmelts cannot be thermally crosslinked, since the very act ofhotmelt processing imposes high temperatures, which would cause gellingduring the operation. In general, therefore, acrylate hotmelts are UVcrosslinked or crosslinked by means of electron beams (EB). Examples ofUV and/or EB crosslinking are U.S. Pat. No. 5,194,455 or DE 27 43 979 orU.S. Pat. No. 5,073,611.

In U.S. Pat. No. 5,877,261 acrylate hotmelts are crosslinked withblocked polyisocyanates. The shelf life of these polymers, however, islimited. Moreover, crosslinking on the backing material requires the useof very high temperatures, which result in damage to the backing. Onespecific example is the drying-out of release papers; another example isthe partial melting of PP or PE backing materials.

There is therefore a need for a thermal crosslinking process foracrylate hotmelt PSAs in which crosslinking is not initiated untilduring or after coating and so no crosslinking reactions can occur inthe hotmelt operation.

It is an object of the invention to provide a polyacrylate PSA which canbe concentrated to a hotmelt and can be processed from the melt, withthermal crosslinking being possible after or during the coatingoperation. The crosslinking reaction ought preferably to take placethermally on the backing material, and to raise the shear strength ofthe PSA.

This object is achieved by means of a polyacrylate pressure-sensitiveadhesive as claimed in claim 1 and also by an associated process forpreparing polyacrylate hotmelt pressure-sensitive adhesives as claimedin claim 5, and by the use of these products as claimed in claim 13. Thesubclaims relate to advantageous developments of the invention.

The invention accordingly provides polyacrylate pressure-sensitiveadhesives which essentially comprise a polymer formed from

a) a comonomer mixture comprising

-   -   a1) acrylic acid and/or acrylic esters of the following formula        CH₂═C(R¹)(COOR²),    -    where R¹═H or CH₃ and R² is an alkyl chain having 1-20 carbon        atoms, at 55%-99% by weight, based on component (a),    -   a2) olefinically unsaturated monomers having functional groups,        specifically in particular having hydroxyl groups, sulfonic acid        groups, ester groups, ether groups, anhydride groups, epoxy        groups, amide groups, amino groups, having aromatic,        heteroaromatic and/or heterocyclic groups, at 0-30% by weight,        based on component (a),    -   a3) acrylate or methacrylate having at least one functional        group at 1%-15% by weight, based on component (a), which is        capable of reacting with a photochemically generated base b),        with or without addition of a catalyst,        the polymer being thermally crosslinked at least partly with a        base b) in a fraction of 0.01%-25% by weight, based on the        overall polymer mixture.

In one very preferred version use is made for the monomers a1) ofacrylic monomers, which include acrylic and methacrylic esters withalkyl groups consisting of 4 to 14 carbon atoms, preferably 4 to 9carbon atoms. Specific examples, without wishing to be restricted bythis enumeration, are n-butyl acrylate, n-pentyl acrylate, n-hexylacrylate, n-heptyl acrylate, n-octyl acrylate, n-nonyl acrylate, laurylacrylate, stearyl acrylate, behenyl acrylate, and their branchedisomers, such as 2-ethylhexyl acrylate, for example. Further classes ofcompound for use, which may likewise be added in small amounts undera1), are methyl methacrylates, cyclohexyl methacrylates and isobornylmethacrylates.

In one further preferred version use is made for the monomers a2) ofvinyl esters, vinyl ethers, vinyl halides, vinylidene halides and vinylcompounds having aromatic rings and heterocycles in a position. Here aswell mention may be made, nonexclusively, of some examples: vinylacetate, vinylformamide, vinylpyridine, ethyl vinyl ether, vinylchloride, vinylidene chloride and acrylonitrile. In a further verypreferred version use is made for the monomers a2) of monomers havingthe following functional groups: hydroxyl, carboxyl, acid amide,isocyanato or amino groups. These groups are used for controlling theadhesive performance properties, and not as functional groups forcrosslinking with the base that is formed.

In one advantageous variant use is made, for a2), of acrylic monomersconforming to the following general formula

where R₁═H or CH₃ and the radical —OR₂ constitutes or comprises thefunctional group of the pressure-sensitive adhesive and not as afunctional group for crosslinking with the base formed from b).

Particularly preferred examples of component a2) are hydroxyethylacrylate, hydroxypropyl acrylate, hydroxyethyl methacrylate,hydroxypropyl methacrylate, allyl alcohol, maleic anhydride, itaconicanhydride, itaconic acid, acrylamide, benzyl acrylate, benzylmethacrylate, phenyl acrylate, phenyl methacrylate, t-butylphenylacrylate, t-butylphenyl methacrylate, phenoxyethyl acrylate,phenoxyethyl methacrylate, 2-butoxyethyl methacrylate, 2-butoxyethylacrylate, dimethylaminoethyl methacrylate, dimethylaminoethyl acrylate,diethylaminoethyl methacrylate, diethylaminoethyl acrylate, cyanoethylmethacrylate, cyanoethyl acrylate, 6-hydroxyhexyl methacrylate,N-tert-butylacrylamide, N-methylolmethacrylamide,N-(buthoxymethyl)methacrylamide, N-methylolacrylamide,N-(ethoxymethyl)acrylamide, N-isopropylacrylamide, vinylacetic acid,tetrahydrofurfuryl acrylate, β-acryloyloxypropionic acid,trichloracrylic acid, fumaric acid, crotonic acid, aconitic acid anddimethylacrylic acid, this enumeration not being conclusive.

In a further preferred version use is made for component a2) of aromaticvinyl compounds, where the aromatic nuclei may be composed preferably ofC₄ to C₁₈ and may also include heteroatoms. Particularly preferredexamples are styrene, 4-vinylpyridine, N-vinylphthalimide,a-methylstyrene, 3,4-dimethoxystyrene and 4-vinylbenzoic acid, thisenumeration not being conclusive.

For the monomers a3) use is made of monomers which carry a functionalgroup which is capable of reaction with the base generated from b),where appropriate with the involvement of a catalyst or of anothercrosslinker substance. In preferred versions use is made of comonomershaving at least one carboxylic acid group, one isocyanato group orepoxide group.

In very preferred versions use is made of comonomers such as glycidylmethacrylate, acrylic acid, methacrylic acid or 2-isocyanatoethylmethacrylate.

For the polymerization the monomers are chosen such that the resultingpolymers can be used as PSAs with industrial utility, particularly suchthat the resulting polymers possess pressure-sensitive adhesionproperties in accordance with the “Handbook of Pressure SensitiveAdhesive Technology” by Donatas Satas (van Nostrand, New York 1989). Forthese applications the static glass transition temperature of theresulting polymer is advantageously below 15° C.

As photobase generators it is preferred to use O-acyl oximes, anilidederivates, ammonium salts or organometallic compounds which under UVirradiation liberate a base.

In one preferred version O-acyl oximes are used as photobase generators.

Depicted below are a number of preferred variants, without any wish tobe restricted to these compounds.

The PSAs of the invention are crosslinked by means of the base obtainedfrom the photobase generator, in accordance with the following scheme:

The amount of photobase generators added is guided by the amount offunctions (in this case, epoxide functions) which are to be used tocrosslink the acrylate PSA. In a very preferred case equimolar amountsare added to the molar fraction of epoxide-containing comonomers. Inthis context it is necessary to bear in mind that, by virtue of itschemical structure, a photobase generator may also liberate twomolecules of amine (in this case, benzylamine). Moreover, one moleculeof amine, or benzylamine, reacts with two polymers in order to effect alinking reaction. Via the amount of photobase generator it is thereforepossible to vary the number of linkage sites to be produced. It is alsonecessary to bear in mind, with regard to the amount of photobasegenerator added, that the yield of the resulting amine after irradiationis likewise limited. The quantum yield of amine formation depends on theone hand on the chemical structure of the photobase generator and on theother on the polymer matrix.

A further influencing factor is the crosslinking efficiency. Thebenzylamine liberated is likewise able to react with two epoxidefunctions of a polymer chain, so that no crosslinking takes place as aresult of this reaction. This influencing factor must also be borne inmind with respect to the amount of photobase generator added.

Besides the free O-acyl oximes it is also possible to introducephotobase generators by way of the polymer. In this case, in onepreferred version, O-acyl oximes or similar compounds having a vinylicdouble bond are copolymerized into the acrylate PSA or are producedalong the polymer chain. Preferred reactive groups available areacrylate, methacrylate and terminal vinyl compounds. These compounds canbe employed in the sence of the monomers a2). In this particular casethere is no need to add free photobase generators. In order to achieveefficient crosslinking with this method, however, it is advisable to adda crosslinking component as well. Suitability in this case is possessedby groups having a functionality of at least two and carrying a group Xwhich is capable of reaction with the generated base. In certainparticularly preferred versions hydroquinone, difunctional epoxides ordifunctional carboxylic acids are added.

Formanilide derivatives as well can be added as photobase generators. Inthis case irradiation with UV light liberates amine compounds, which arethen able to react, for example, with carboxylic acid groups of thepolymer chains and hence to bring about crosslinking.

Depicted below are a number of specific examples, without any wish thatthis enumeration should constitute any restriction.

Besides the group of the anilides it is also possible to use ammoniumsalts as photobase generators, these salts generating free amines whenirradiated with UV light. Listed below are a number of specificexamples, without wishing to be restricted thereby.

In addition to the group of the anilides and ammonium salts as photobasegenerators it is also possible to use photobases which liberatepolyfunctional thiols after UV irradiation. A specific example isdepicted below.

As well as the above photobase generators there exists a multiplicity oforganometallic compounds which likewise liberate bases under UVirradiation.[Co(III)(NH₂C₃H₇)₅Br]² ⁺   (Xa)[Co(III)(NH₃)₆]³⁺B(C₆H₅)₄ ⁻2B(C₆H₅)₄   (Xb)

As well as amines, however, imidazoles as well can be generated by UVirradiation using, for example, the following photobase generator.

Further difunctional amines can be produced using the followingcompounds.

For crosslinking with UV light, in one very advantageous version of theinvention, further UV-absorbing photoinitiators are added to thepolyacrylate PSAs. Useful photoinitiators whose use is very beneficialare benzoin ethers, such as benzoin methyl ether and benzoin isopropylether, substituted acetophenones, such as 2,2-diethoxyacetophenone(available as Irgacure 651® from Ciba Geigy®),2,2-dimethoxy-2-phenyl-1-phenylethanone, dimethoxyhydroxyacetophenone,substituted ketols, such as 2-methoxy-2-hydroxypropiophenone, aromaticsulfonyl chlorides, such as 2-naphthylsulfonyl chloride, and photoactiveoximes, such as 1-phenyl-1,2-propanedione 2-(O-ethoxycarbonyl)oxime, forexample. In one very preferred version benzophenone is added.

The abovementioned photoinitiators and others which can be used, andothers of the Norrish I or Norrish II type, may contain the followingradicals: benzophenone, aceto-phenone, benzil, benzoin,hydroxyalkylphenone, phenyl cyclohexyl ketone, anthraquinone,trimethylbenzoylphosphine oxide, methylthiophenyl morpholine ketone,amino ketone, azo benzoin, thioxanthone, hexarylbisimidazole, triazineor fluorenone, it being possible for each of these radicals additionallyto be substituted by one or more halogen atoms and/or one or morealkyloxy groups and/or one or more amino groups or hydroxyl groups. Arepresentative overview is given by Fouassier: “Photoinitiatation,Photopolymerization and Photocuring: Fundamentals and Applications”,Hanser-Verlag, Munich 1995. For supplemental information Carroy et al.in “Chemistry and Technology of UV and EB Formulation for Coatings, Inksand Paints”, Oldring (ed.), 1994, SITA, London can be consulted.

The polyacrylate PSAs of the invention are obtained by means of anassociated process of the invention, in which polymers are prepared from

a) a comonomer mixture comprising

-   -   a1) acrylic acid and/or acrylic esters of the following formula        CH₂═C(R¹)(COOR²),    -    where R¹═H or CH₃ and R² is an alkyl chain having 1-20 carbon        atoms, at 55%-99% by weight, based on component (a),    -   a2) olefinically unsaturated monomers having functional groups,        specifically in particular having hydroxyl groups, sulfonic acid        groups, ester groups, ether groups, anhydride groups, epoxy        groups, amide groups, amino groups, having aromatic,        heteroaromatic and/or heterocyclic groups, at 0-30% by weight,        based on component (a),    -   a3) acrylate or methacrylate having at least one functional        group at 1%-15% by weight, based on component (a), which is        capable of reacting with the base generated by b), with or        without a catalyzing compound,    -   and b) at least one photobase generator at 0.01%-25% by weight,        based. on the overall polymer mixture,        where b) is incorporated by mixing or copolymerization and where        the solvent-free polymer or the polymer substantially freed from        solvent is coated with the photobase generator, in a hotmelt        process, onto a backing, and during or after its coating is        irradiated with UV light, thereby generating a base        photochemically, and the composition is subsequently crosslinked        thermally by reaction at least of component a3) with the base.

The solvent—if such is used—is preferably removed with heating underreduced pressure. In one particularly advantageous embodiment thepolymer can be laid onto a film of water and subsequently transferred tothe backing material, with the water preferably contributing to thecrosslinking of the PSA. This is described in more detail below.

For the preparation of the polyacrylate PSAs first of all conventionalradical polymerizations or controlled radical polymerizations arecarried out for the purpose of preparing the polymers that aresubsequently to be crosslinked. For the polymerizations which proceed bya radical mechanism it is preferred to use initiator systems whichadditionally include further radical initiators for the polymerization,especially thermally decomposing radical-forming azo or peroxoinitiators. In principle, however, all customary initiators that areknown for acrylates are suitable for this pupose. The production ofC-centered radicals is described in Houben Weyl, Methoden derOrganischen Chemie, Vol. E 19a, pp. 60-147. These methods arepreferentially employed analogously.

Examples of radical sources are peroxides, hydroperoxides and azocompounds; as a number of nonexclusive examples of typical radicalinitiators mention may be made here of potassium peroxodisulfate,dibenzoyl peroxide, cumene hydroperoxide, cyclohexanone peroxide,di-t-butyl peroxide, azodiisobutyronitrile, cyclohexylsulfonyl acetylperoxide, diisopropyl percarbonate, t-butyl peroctoate and benzpinacol.In one very preferred version a radical initiator used is1,1′-azobis(cyclohexanecarbononitrile) (Vazo 88™ from DuPont).

The average molecular weights M_(n) of the PSAs formed in the controlledradical polymerization are chosen such that they are situated within arange from 20 000 and 2 000 000 g/mol. Specifically for further use ashotmelt PSAs, PSAs having molecular weights M_(n) of from 100 000 to 500000 g/mol are preferred. The average molecular weight is determined byway of size exclusion chromatography (SEC) or matrix-assisted laserdesorption/ionization—mass spectrometry (MALDI-MS).

The polymerization may be conducted in bulk, in the presence of anorganic solvent, in the presence of water, or in mixtures of organicsolvents and water. The aim is to minimize the amount of solvent used.Suitable organic solvents or mixtures of solvents are pure alkanes(hexane, heptane, octane, isooctane), aromatic hydrocarbons (benzene,toluene, xylene), esters (ethyl acetate, propyl acetate, butyl acetateor hexyl acetate), halogenated hydrocarbons (chlorobenzene), alkanols(methanol, ethanol, ethylene glycol, ethylene glycol monomethyl ether)and ethers (diethyl ether, dibutyl ether) or mixtures thereof.

A water-miscible or hydrophilic cosolvent may be added to the aqueouspolymerization reactions in order to ensure that in the course ofmonomer conversion the reaction mixture is in the form of a homogeneousphase. Cosolvents which can be used with advantage for the presentinvention are selected from the following group, consisting of aliphaticalcohols, glycols, ethers, glycol ethers, pyrrolidines,N-alkylpyrrolidinones, N-alkylpyrrolidones, polyethylene glycols,polypropylene glycols, amides, carboxylic acids and salts thereof,esters, organic sulfides, sulfoxides, sulfones, alcohol derivatives,hydroxy ether derivatives, amino alcohols, ketones and the like, andalso derivatives and mixtures thereof.

Depending on conversion and temperature, the polymerization time amountsto between 4 and 72 hours. The higher the reaction temperature that canbe chosen, in other words the higher the thermal stability of thereaction mixture, the lower the reaction time that can be chosen.

For initiating the polymerization the introduction of heat is essentialfor the thermally decomposing initiators. For the thermally decomposinginitiators the polymerization can be initiated by heating at 50-160° C.,depending on initiator type.

For the preparation of polyacrylate PSAs having a narrow molecularweight distribution suitability is also possessed by controlled radicalpolymerization methods. In that case it is preferred, for the purpose ofthe polymerization, to use a control reagent of the general formula:

in which

R and R′, chosen independently of one another or identical, represent

-   -   branched and unbranched C₁- to C₁₈ alkyl radicals, C₃- to C₁₈        alkenyl radicals or C₃- to C₁₈ alkynyl radicals;    -   H or C₁- to C₁₈ alkoxy;    -   C₁- to C₁₈ alkyl radicals; C₃- to C₁₈ alkenyl radicals; C₃- to        C₁₈ alkynyl radicals that are substituted by at least one OH        group or halogen atom or silyl ether;    -   C₂-C₁₈ heteroalkyl radicals having at least one oxygen atom        and/or one NR′ group in the carbon chain;    -   C₁-C₁₈ alkyl radicals, C₃-C₁₈ alkenyl radicals or C₃-C₁₈ alkynyl        radicals substituted by at least one ester group, amine group,        carbonate group, cyano, isocyanto and/or epoxide group and/or by        sulfur;    -   C₃-C₁₂ cycloalkyl radicals;    -   C₆-C₁₈ aryl radicals or benzyl radicals;    -   hydrogen.

Control reagents of type (I) are composed, in one more-preferredversion, of the following, further-restricted compound:

halogens in this case are preferably F, Cl, Br or I, more preferably Cland Br. Suitable alkyl, alkenyl and alkynyl radicals in the varioussubstituents include outstandingly not only linear chains but alsobranched chains.

Examples of alkyl radicals which contain 1 to 18 carbon atoms aremethyl, ethyl, propyl, isopropyl, butyl, isobutyl, t-butyl, pentyl,2-pentyl, hexyl, heptyl, octyl, 2-ethylhexyl, t-octyl, nonyl, decyl,undecyl, tridecyl, tetradecyl, hexadecyl and octadecyl.

Examples of alkenyl radicals having 3 to 18 carbon atoms are propenyl,2-butenyl, 3-butenyl, isobutenyl, n-2,4-pentadienyl, 3-methyl-2-butenyl,n-2-octenyl, n-2-dodecenyl, isododecenyl and oleyl.

Examples of alkynyl having 3 to 18 carbon atoms are propynyl, 2-butynyl,3-butynyl, n-2-octynyl and n-2-octadecynyl.

Examples of hydroxy-substituted alkyl radicals are hydroxypropyl,hydroxybutyl or hydroxyhexyl.

Examples of halogen-substituted alkyl radicals are dichlorobutyl,monobromobutyl or trichlorohexyl.

One suitable C₂-C₁₈ heteroalkyl radical having at least one oxygen atomin the carbon chain is for example —CH₂—CH₂—O—CH₂—CH₃.

Examples of C₃-C₁₂ cycloalkyl radicals include cyclopropyl, cyclopentyl,cyclohexyl or trimethylcyclohexyl.

Examples of C₆-C₁₈ aryl radicals include phenyl, naphthyl, benzyl,4-tert-butylbenzyl or further substituted phenyl, such as ethyl,toluene, xylene, mesitylene, isopropylbenzene, dichlorobenzene orbromotoluene.

The listings above serve only as examples of the respective groups ofcompounds, and possess no claim to completeness.

Also suitable are compounds of the following types

where R″ may comprise either the radicals R or R′.

In the case of the conventional RAFT process polymerization is generallycarried out only up to low conversions (WO 98/01478 A1) in order toproduce very narrow molecular weight distributions. As a result of thelow conversions, however, these polymers cannot be used as PSAs and inparticular not as hotmelt PSAs, since the high fraction of residualmonomers adversely affects the adhesive performance properties; theresidual monomers contaminate the solvent recyclate in the concentrationoperation; and the corresponding self-adhesive tapes would exhibit veryhigh outgassing. In order to circumvent this drawback of lowconversions, the polymerization in one particularly preferred version isinitiated two or more times.

As a further controlled radical polymerization method it is possible tocarry out nitroxide-controlled polymerizations. For radicalstabilization, in a favorable procedure, use is made of nitroxides oftype (Va) or (Vb):

where R³, R⁴, R⁵, R⁶, R⁷, R⁸, R⁹ and R¹⁰ independently of one anotherdenote the following compounds or atoms:

-   -   i) halides, such as chlorine, bromine or iodine, for example,    -   ii) linear, branched, cyclic and heterocyclic hydrocarbons        having 1 to 20 carbon atoms, which may be saturated, unsaturated        or aromatic,    -   iii) esters —COOR¹¹, alkoxides —OR¹² and/or phosphonates        —PO(OR¹³)₂, where R¹¹, R¹² or R¹³ stand for radicals from group        ii).

Compounds of the (Va) or (Vb) may also be attached to polymer chains ofany kind (primarily such that at least one of the abovementionedradicals constitutes a polymer chain of this kind) and may thus be used,for example, as macroradicals or macroregulators for synthesizing theblock copolymers.

Controlled regulators become more preferred for the polymerization ofcompounds of the type:

-   -   2,2,5,5-tetramethyl-1-pyrrolidinyloxyl (PROXYL),        3-carbamoyl-PROXYL, 2,2-dimethyl-4,5-cyclohexyl-PROXYL,        3-oxo-PROXYL, 3-hydroxylimine-PROXYL, 3-aminomethyl-PROXYL,        3-methoxy-PROXYL, 3-t-butyl-PROXYL, 3,4-di-t-butyl-PROXYL    -   2,2,6,6-tetramethyl-1-piperidinyloxy pyrrolidinyloxyl (TEMPO),        4-benzoyloxy-TEMPO, 4-methoxy-TEMPO, 4-chloro-TEMPO,        4-hydroxy-TEMPO, 4-oxo-TEMPO, 4-amino-TEMPO,        2,2,6,6-tetraethyl-1-piperidinyloxyl,        2,2,6-trimethyl-6-ethyl-1-piperidinyloxyl    -   N-tert-butyl 1-phenyl-2-methylpropyl nitroxide    -   N-tert-butyl 1-(2-naphthyl)-2-methylpropyl nitroxide    -   N-tert-butyl 1-diethylphosphono-2,2-dimethylpropyl nitroxide    -   N-tert-butyl 1-dibenzylphosphono-2,2-dimethylpropyl nitroxide    -   N-(1-phenyl-2-methyl propyl) 1-diethylphosphono-1-methylethyl        nitroxide    -   di-t-butyl nitroxide    -   diphenyl nitroxide    -   t-butyl t-amyl nitroxide

U.S. Pat. No. 4,581,429 A itself discloses a controlled-growth radicalpolymerization process which uses as its initiator a compound of theformula R′R″N—O—Y, in which Y is a free radical species which is able topolymerize unsaturated monomers. The reactions, however, generally havelow conversions. A particular problem is the polymerization ofacrylates, which proceeds only to very low yields and molar masses. WO98/13392 A1 describes open-chain alkoxyamine compounds which have asymmetrical substitution pattern. EP 735 052 A1 discloses a process forpreparing thermoplastic elastomers having narrow molar massdistributions. WO 96/24620 A1 describes a polymerization process inwhich very specific radical compounds, such as phosphorus-containingnitroxides based on imidazolidine, for example, are used. WO 98/44008 A1discloses specific nitroxyls based on morpholines, piperazinones andpiperazinediones. DE 199 49 352 A1 describes heterocyclic alkoxyaminesas regulators in controlled-growth radical polymerizations.Corresponding further developments of the alkoxyamines or of thecorresponding free nitroxides improve the efficiency for the preparationof polyacrylates (Hawker, contribution to the National Meeting of theAmerican Chemical Society, Spring 1997; Husemann, contribution to theIUPAC World Polymer Meeting 1998, Gold Coast).

As a further control polymerization it is possible with advantage tosynthesize the polyacrylate PSAs using atom transfer radicalpolymerization (ATRP), in which case preferred initiators used aremonofunctional or difunctional secondary or tertiary halides and thehalide(s) is (are) abstracted using complexes of Cu, Ni, Fe, Pd, Pt, Ru,Os, Rh, Co, Ir, Ag or Au (EP 0 824 111 A1; EP 826 698A1; EP 824 110A1;EP 841 346A1; EP 850 957 A1). The various possibilities of ATRP arefurther described in U.S. Pat. No. 5,945,491 A, U.S. Pat. No. 5,854,364A and U.S. Pat. No. 5,789,487 A.

For further development it is possible to admix resins to thepolyacrylate PSAs. Tackifying resins for addition that can be usedinclude, without exception, all of the tackifier resins that are alreadyknown and are described in the literature. Representatives that may bementioned include pinene resins, indene resins and rosins, theirdisproportionated, hydrogenated, polymerized and/or esterifiedderivatives and salts, the aliphatic and aromatic hydrocarbon resins,terpene resins and terpene-phenolic resins and also C5, C9 and otherhydrocarbon resins. Any desired combinations of these and further resinsmay be used in order to adjust the properties of the resultant adhesivein accordance with what is desired. Generally speaking it is possible touse any resins which are compatible with (soluble in) the correspondingpolyacrylate, reference being made in particular to all aliphatic,aromatic and alkylaromatic hydrocarbon resins, hydrocarbon resins basedon single monomers, hydrogenated hydrocarbon resins, functionalhydrocarbon resins, and natural resins. Express reference is made to thedepiction of the state of the art in the “Handbook of Pressure SensitiveAdhesive Technology” by Donatas Satas (van Nostrand, 1989).

Additionally it is possible optionally to add plasticizers, fillers(e.g., fibers, carbon black, zinc oxide, titanium dioxide, chalk, solidor hollow glass spheres, microspheres of other materials, silica,silicates), nucleators, expandants, compounding agents and/or aginginhibitors, in the form for example of primary and secondaryantioxidants or in the form of light stabilizers.

The polyacrylate PSAs can be applied from solution or from the melt tothe backing material. For application from the melt the solvent ispreferably stripped off in a concentrating extruder under reducedpressure, for which purpose use may be made, for example, ofsingle-screw or twin-screw extruders, which preferably distill off thesolvent in identical or different vacuum stages and possess a feedpreheater.

In one preferred version of the invention—especially if water is neededas the catalyst or for crosslinking—the PSAs are deposited on a film ofwater. In one preferred version of the invention the film of water islocated on a metal roll.

The metal roll may be equipped with an effective cooling system, inorder to transport away the considerable quantities of heat. For certaincases, however, it may also be of advantage to heat the film of water bymeans of a heating system.

To prevent corrosion it is commonly coated with a protective coat. Thiscoat is preferably selected such that it is effectively wetted by thecontact medium. Generally the surface is conductive. It may also be morefavorable, however, to coat it with one or more coats of insulating orsemiconducting material.

Additionally the metal roll may be macroscopically smooth or may have aslightly textured surface. It has been found appropriate for it to havea surface texture, in particular a surface roughening. This allowswetting by the contact medium to take place more effectively.

The process proceeds to particularly good effect if the metal roll istemperature-controllable, preferably in a range of −10 C to 200 C, verypreferably from 5 C to 70 C.

As the contact liquid it is advantageous to use water. In a furthervariant, which is outstandingly suitable for the purposes of theinvention, substances are added to the water contact medium that are atleast partially soluble therein. For a water contact medium, appropriateadditives include, for example, alkyl alcohols such as ethanol,propanol, butanol and hexanol, without wishing to be restricted in theselection of alcohols as a result of these examples. Also highlyadvantageous are, in particular, longer-chain alcohols, polyglycols,ketones, amines, carboxylates, sulfonates and the like.

It has proven very advantageous to use as the contact medium a liquidwhich if desired acquires additives for additional functions. Thesefunctions include the increase of surface wetting.

A reduction in the surface tension may also be achieved by adding smallamounts of nonionic and/or anionic and/or cationic surfactants to thecontact medium. The most simple way of achieving this is by usingcommercial washing compositions or soap solutions, preferably in aconcentration of a few g/l in water, as contact medium. Particularlysuitable compounds are specific surfactants which can be used even at alow concentration. Examples thereof that may be mentioned includesulfonium surfactants (e.g. β-di(hydroxyalkyl)sulfonium salt), and also,for example, ethoxylated nonylphenyl-sulfonic acid ammonium salts. Hereparticular reference may be made to the state of the art under“surfactants” in Ullmann's Encyclopedia of Industrial Chemistry, SixthEdition, 2000 Electronic Release, Wiley-VCH, Weinheim 2000.

Where water is used as the contact medium it is possible to proceedoutstandingly by running a second roller, advantageously having awettable or absorbent surface, through a bath containing the contactmedium, said roller then becoming wetted by or impregnated with thecontact medium and applying a film of said contact medium by contactwith the chill roll.

In one very preferred version of the invention the pressure-sensitiveadhesive is irradiated with UV light directly on the film of water ofthe metal roll.

In a procedure which is advantageous for the process operation takesplace by means of brief ultraviolet irradiation within a wavelengthrange from 200 to 400 nm, depending on the photobase generator used,using in particular high-pressure or medium-pressure mercury lamps withan output of from 80 to 240 W/cm. The irradiation intensity is adaptedto the respective quantum yield of the photobase generator. After theoperation of coating on the film of water and UV irradiation, thepolyacrylate PSA is transferred from the film of water to a backingmaterial.

Backing materials used for the PSA, such as for PSA tapes, for example,are the materials that are customary and familiar to the skilled worker,such as films (polyesters, PET, PE, PP, BOPP, PVC), nonwovens, foams,woven fabrics and woven films, and also release paper (glassine, HDPE,LDPE). This enumeration is not conclusive.

The guided supply of the backing, i.e., for example, of therelease-coated materials such as papers, films or nonwovens, isadvantageously performed under a certain contact pressure.

After the operation of coating the adhesive on the backing the PSA tape,in one preferred version, is further irradiated with UV light. In oneadvantageous variant this procedure takes place by means of briefultraviolet radiation in a wavelength range from 200 to 400 nm,depending on the photobase generator used, in particular usinghigh-pressure or medium-pressure mercury lamps with an output of from 80to 240 W/cm. The intensity of irradiation is adapted to the respectivequantum yield of the photobase generator.

It can be of great advantage for the process, especially if water isrequired as catalyst or for crosslinking, if the PSA tape is in contactwith water during or after UV crosslinking. In one variant the PSA tapeis run through a water bath, while in another variant the PSA tape isguided through a controlled-climate chamber featuring high air humidity.It is also possible to store the PSA tapes in a controlled-climatechamber featuring high air humidity.

In one very preferred version of the process the PSA tape is heatedagain after it has been UV-irradiated and moistened by means of water.In one very preferred version this can be done in a drying tunnel. Toinitiate the crosslinking reaction the PSA tape is heated to at least80° C., in one very preferred variant to at least 100° C. Heating can becarried out using different sources, all of which are able to emit heat.In one very preferred version use is made of IR lamps. In very simpleand practical versions, hot-air fans or other heat machines whichgenerate by means of electrical energy are also suitable, however.

In principle it is also possible to subject the polyacrylate PSA tocrosslinking with electron beams as well. Typical irradiation equipmentwhich may be employed includes linear cathode systems, scanner systemsand segmented cathode systems, where the equipment in question compriseselectron beam accelerators. A detailed description of the state of theart and the most important process parameters can be found in Skelhorne,Electron Beam Processing, in Chemistry and Technology of UV and EBformulation for Coatings, Inks and Paints, Vol. 1, 1991, SITA, London.The typical acceleration voltages are situated in the range between 50kV and 500 kV, preferably 80 kV and 300 kV. The irradiation dosesemployed range between 5 to 150 kGy, in particular between 20 and 100kGy.

In a further version according to the invention the polyacrylate PSA isin principle subjected only to structured irradiation. A feature of thisprocess for producing structured polyacrylates by means of structuredcrosslinking of polyacrylate mixtures is that the base polymer mixtureis irradiated with ultraviolet light such that only particular regionsof the polymer mixture are exposed to UV radiation.

The process for production can be conducted in particular such that thebase polymer mixture is irradiated with ultraviolet light through aperforated mask in such a way that only particular regions of thepolymer mixture are exposed to UV radiation.

Alternatively the structuring of the polymer mixture to be cured can beachieved by using, instead of the perforated mask, a film whose surfacearea includes regions of different UV-light transparency, so thatcertain regions of the polymer mixture are exposed to differentintensities of UV radiation.

The process outlined above is illustrated below with reference to FIG. 1

FIG. 1: Diagrammatic representation of a structured crosslinking bymeans of UV irradiation through a perforated mask.

FIG. 1 depicts the irradiation of the acrylate composition (2) through aperforated mask (1), the acrylate composition (2) being located on thebacking (3). According to the main claim the acrylate composition (1) isadmixed with a photobase generator, which as a result of UV light (4)and water, and at high temperatures (ΔT), performs partial crosslinkingof epoxide groups of the polymer (2). The ultraviolet rays (4) are ableto penetrate the mask (1) only in the region of the perforations (11),so that following irradiation the resulting situation is that depictedin the bottom part of the figure: the PSA (2) has hard segments of highcrosslinking (21) and also uncrosslinked, soft segments (22).

The polymer chains at the edges of the hard regions reach into the softregions, and so the hard regions, which are of inherently highviscosity, are linked to the soft regions and therefore hinder theselatter regions in terms of their mobility, thereby raising thestructural strength of the adhesive. These hard segments, moreover,increase the cohesion of the PSA. In contrast, the soft segments (22)result in readier flow of the adhesive on the substrate and,accordingly, increase the bond strength and the tack. A large influenceon the adhesive performance properties is exerted by the percentagefraction of the irradiated area and also by the size of the segmentsproduced.

The invention further comprises the use of the polyacrylate as apressure-sensitive adhesive, in particular its use as apressure-sensitive adhesive for an adhesive tape, in which case thepressure-sensitive acrylate adhesive is present in the form of aone-sided or double-sided film on a backing sheet.

EXAMPLES

The following exemplary experiments are intended to illustrate thecontent of the invention, without any intention that the inventionshould be unnecessarily restricted as a result of the choice of theexamples.

Test Methods

The test methods set out below were used to characterize thepolyacrylate compositions and their crosslinked products:

Shear Strength (Test A1, A2)

A strip of the adhesive tape 13 mm wide was applied to a smooth steelsurface which had been cleaned. The area of application was 20 mm×13 mm(length×width). The following procedure was then adopted:

Test A1: at room temperature a 1 kg weight was attached to the adhesivetape and the time taken for the weight to fall was recorded.

Test A2: at 70° C. a 0.5 kg weight was attached to the adhesive tape andthe time taken for the weight to fall was recorded.

The shear withstand times recorded are each reported in minutes, andcorrespond to the average of three measurements.

180° Bond Strength Test (Test B)

A strip 20 mm wide of an acrylate PSA applied as a layer on polyesterwas applied to steel plates which had been cleaned twice with acetoneand once with isopropanol. The PSA strip was pressed onto the substratetwice using a 2 kg weight. Immediately thereafter the adhesive tape waspeeled from the subsrate at an angle of 180° and at 300 mm/min, and theforce required to achieve this was recorded. All measurements wereconducted at room temperature.

The results are reported in N/cm and have been averaged from threemeasurements.

Determination of the Gel Fration (Test C)

After careful drying, the solvent-free samples of adhesive are weldedinto a pouch made of polyethylene nonwoven (Tyvek web). The differencein the sample weights before and after extraction using toluenedetermines the gel index, which is the weight fraction of polymer thatis not soluble in toluene.

Implementation of the Hotmelt process in a Recording Extruder:

The shearing and thermal working of the acrylate hotmelts was carriedout using the Rheomix 610p recording extruder from Haake. The drive unitavailable was the Rheocord RC 300p. The apparatus was controlled usingthe PolyLab System software. The extruder was charged in each case with52 g of pure acrylate PSA (˜80% fill level). The experiments wereconducted with a kneading temperature of 120° C., a rotary speed of 60rpm and a kneading time of 6 hours. Thereafter the specimens weredissolved again, if possible, and the gel index was determined inaccordance with test C.

UV Irradiation

UV irradiation was carried out using a UV unit from Eltosch. The unit isequipped with a medium-pressure Hg UV lamp having an intensity of 120W/cm. The swatches were each run through the unit at a speed of 20m/min, the specimens being irradiated in two or more passes in order toincrease the radiation dose. The UV dose was measured using thePower-Puck from Eltosch. The dose of one irradiation pass amounted toabout 140 mJ/cm² in the UV-B region and 25 mJ/cm² in the UV-C region.

Photobase Generators:

Bis[[(2-nitrobenzyl)oxy]carbonyl]hexane-1,6-diamine (XIIa) was preparedaccording to instructions in Journal of Polymer Science: Part A: PolymerChemistry, Vol. 31, 3013-3020 (1993).

4,4′-[Bis[[(2-nitrobenzyl)oxy]carbonyl]trimethylene]dipiperidine (XIIb)was synthesized according to instructions from Macromolecules 1997, 30,pp. 1304-1310.

Regulator:

Bis-2,2′-phenylethyl thiocarbonate is synthesized starting from2-phenylethyl bromide using carbon disulfide and sodium hydroxide inaccordance with instructions from Synth. Communications 18(13), pp.1531-1536, 1988. Yield after distillation: 72%. ¹H-NMR (CDCl₃) δ (ppm):7.20-7.40 (m, 10 H), 1.53, 1.59 (2×d, 6 H), 3.71, 381 (2×m, 2 H).

Example 1

A 2 L glass reactor conventional for radical polymerizations was chargedwith 40 g of acrylic acid, 360 g of 2-ethylhexyl acrylate, 0.55 g ofbis-2,2′-phenylethyl thiocarbonate and 170 g of acetone. After nitrogengas had been passed through the reactor for 45 minutes, with stirring,the reactor was heated to 58° C. and 0.3 g of azoisobutyronitrile (AIBN,Vazo 64™, DuPont) was added. Subsequently the external heating bath washeated to 75° C. and the reaction was carried out constantly at thisexternal temperature. After a reaction time of 1 h a further 0.3 g ofAIBN was added. Dilution took place after 4 h and 8 h, using 100 g ofacetone each time. After a reaction time of 48 h the reaction wasterminated and cooling took place to room temperature.

Subsequently the polymer was blended in solution with 5% by weight ofbis[[(2-nitro-benzyl)oxy]carbonyl]hexane-1,6-diamine and thenconcentrated in a vacuum drying oven at 80° C. and a pressure of 10torr. Subsequently the hotmelt process was carried out in the recordingextruder. For crosslinking, the acrylate hotmelt was coated at 50 g/m²onto a PET film, using a heatable laboratory roll coater, irradiatedwith UV light in a number of passes, in accordance with the methoddescribed above, and then conditioned at 140° C. for 20 minutes. Finallythe specimens were tested according to methods A and B.

Example 2

The procedure of Example 1 was repeated. The photobase generator addedwas 5% by weight of4,4′-[bis[[(2-nitrobenzyl)oxy]carbonyl]trimethylene]dipiperidine.

Example 3

A 2 L glass reactor conventional for radical polymerizations was chargedwith 30 g of acrylic acid, 100 g of n-butyl acrylate, 270 g of2-ethylhexyl acrylate, 0.55 g of bis-2,2′-phenylethyl thiocarbonate and170 g of acetone. After nitrogen gas had been passed through the reactorfor 45 minutes, with stirring, the reactor was heated to 58° C. and 0.3g of azoisobutyronitrile (AIBN, Vazo 64™, DuPont) was added.Subsequently the external heating bath was heated to 75° C. and thereaction was carried out constantly at this external temperature. Aftera reaction time of 1 h a further 0.3 g of AIBN was added. Dilution tookplace after 4 h and 8 h, using 100 g of acetone each time. After areaction time of 48 h the reaction was terminated and cooling took placeto room temperature.

Subsequently the polymer was blended in solution with 3% by weight ofbis[[(2-nitro-benzyl)oxy]carbonyl]hexane-1,6-diamine and thenconcentrated in a vacuum drying oven at 80° C. and a pressure of 10torr. Subsequently the hotmelt process was carried out in the recordingextruder. For crosslinking, the acrylate hotmelt was coated at 50 g/m²onto a PET film, using a heatable laboratory roll coater, irradiatedwith UV light in a number of passes, in accordance with the methoddescribed above, and then conditioned at 140° C. for 20 minutes. Finallythe specimens were tested according to methods A and B.

Example 4

A 2 L glass reactor conventional for radical polymerizations was chargedwith 8 g of glycidyl methacrylate, 100 g of isobornyl acrylate, 292 g of2-ethylhexyl acrylate, 0.55 g of bis-2,2′-phenylethyl thiocarbonate and170 g of acetone: special-boiling-point spirit 60/95 (1:1). Afternitrogen gas had been passed through the reactor for 45 minutes, withstirring, the reactor was heated to 58° C. and 0.3 g ofazoisobutyronitrile (AIBN, Vazo 64™, DuPont) was added. Subsequently theexternal heating bath was heated to 75° C. and the reaction was carriedout constantly at this external temperature. After a reaction time of 1h a further 0.3 g of AIBN was added. Dilution took place after 4 h and 8h, using 100 g of spirit in each case. After a reaction time of 48 h thereaction was terminated and cooling took place to room temperature.

Subsequently the polymer was blended in solution with 3% by weight ofbis[[(2-nitro-benzyl)oxy]carbonyl]hexane-1,6-diamine and thenconcentrated in a vacuum drying oven at 80° C. and a pressure of 10torr. Subsequently the hotmelt process was carried out in the recordingextruder. For crosslinking, the acrylate hotmelt was coated at 50 g/m²onto a PET film, using a heatable laboratory roll coater, the specimenwas stored at 60° C. and 98% humidity for one day, irradiated with UVlight in a number of passes, in accordance with the method describedabove, and then conditioned at 140° C. for 20 minutes. Finally thespecimens were tested according to methods A and B.

Example 5

A 2 L glass reactor conventional for radical polymerizations was chargedwith 12 g of 2-isocyanatoethyl methacrylate, 100 g of isobornylacrylate, 288 g of 2-ethylhexyl acrylate, 0.55 g of bis-2,2′-phenylethylthiocarbonate and 170 g of acetone: special-boiling-point spirit 60/95(1:1). After nitrogen gas had been passed through the reactor for 45minutes, with stirring, the reactor was heated to 58° C. and 0.3.g ofazoisobutyronitrile (AIBN, Vazo 64™, DuPont) was added. Subsequently theexternal heating bath was heated to 75° C. and the reaction was carriedout constantly at this external temperature. After a reaction time of 1h a further 0.3 g of AIBN was added. Dilution took place after 4 h and 8h, using 100 g of spirit. After a reaction time of 48 h the reaction wasterminated and cooling took place to room temperature.

Subsequently the polymer was blended in solution with 4% by weight ofbis[[(2-nitro-benzyl)oxy]carbonyl]hexane-1,6-diamine and thenconcentrated in a vacuum drying oven at 80° C. and a pressure of 10torr. Subsequently the hotmelt process was carried out in the recordingextruder. For crosslinking, the acrylate hotmelt was coated at 50 g/m²onto a PET film, using a heatable laboratory roll coater, irradiatedwith UV light in a number of passes, in accordance with the methoddescribed above, and then conditioned at 140° C. for 30 minutes. Finallythe specimens were tested according to methods A and B.

Example 6

A 2 L glass reactor conventional for radical polymerizations was chargedwith 28 g of acrylic acid, 80 g of methyl acrylate, 292 g of2-ethylhexyl acrylate, 0.40 g of bis-2,2′-phenylethyl thiocarbonate and170 g of acetone. After nitrogen gas had been passed through the reactorfor 45 minutes, with stirring, the reactor was heated to 58° C. and 0.3g of azoisobutyronitrile (AIBN, Vazo 64™, DuPont) was added.Subsequently the external heating bath was heated to 75° C. and thereaction was carried out constantly at this external temperature. Aftera reaction time of 1 h a further 0.3 g of AIBN was added. Dilution tookplace after 4 h and 8 h, using 100 g of acetone each time. After areaction time of 48 h the reaction was terminated and cooling took placeto room temperature.

Subsequently the polymer was blended in solution with 8% by weight ofbis[[(2-nitro-benzyl)oxy]carbonyl]hexane-1,6-diamine and thenconcentrated in a vacuum drying oven at 80° C. and a pressure of 10torr. Subsequently the hotmelt process was carried out in the recordingextruder. For crosslinking, the acrylate hotmelt was coated at 50 g/m²onto a PET film, using a heatable laboratory roll coater, lined with asiliconized PET film, masked using a mask having a mesh size of 60 μmand an open area of 40%, irradiated with UV light, and following removalof the mask conditioned subsequently at 120° C. for 10 minutes. Finallythe specimens were tested according to methods A and B.

Reference Example R1

A 2 L glass reactor conventional for radical polymerizations was chargedwith 40 g of acrylic acid, 360 g of 2-ethylhexyl acrylate, 0.55 g ofbis-2,2′-phenylethyl thiocarbonate and 170 g of acetone. After nitrogengas had been passed through the reactor for 45 minutes, with stirring,the reactor was heated to 58° C. and 0.3 g of azoisobutyronitrile (AIBN,Vazo 64™, DuPont) was added. Subsequently the external heating bath washeated to 75° C. and the reaction was carried out constantly at thisexternal temperature. After a reaction time of 1 h a further 0.3 g ofAIBN was added. Dilution took place after 4 h and 8 h, using 100 g ofacetone each time. After a reaction time of 48 h the reaction wasterminated and cooling took place to room temperature.

Subsequently the polymer was concentrated in a vacuum drying oven at 80°C. and a pressure of 10 torr. Subsequently the hotmelt process wascarried out in the recording extruder. For crosslinking, the acrylatehotmelt was coated at 50 g/m² onto a PET film, using a heatablelaboratory roll coater, irradiated with UV light in a number of passes,in accordance with the method described above, and then conditioned at140° C. for 20 minutes. Finally the specimens were tested according tomethods A and B.

Results

Set out below are the results of Examples 1 to 6. Withbis[[(2-nitrobenzyl)oxy]carbonyl]-hexane-1,6-diamine (XIIa) and4,4′-[bis[[(2-nitrobenzyl)oxy]carbonyl]trimethylene]-dipiperidine(XIIb), two different photobase generators were trialed. Additionally,different crosslinking mechanisms were trialed. In Examples 1 to 3crosslinking took place via the carboxylic acid groups of thepolyacrylate PSAs. In Example 4 crosslinking via epoxide groups wasinvestigated, and in Example 5 crosslinking via isocyanate groups. InExample 6 the acrylate hotmelt PSA of the invention was irradiatedselectively, through a mask, so as to generate a structured PSA by wayof the photobase technique.

To investigate their suitability as hotmelt PSAs, all of the exampleswere subjected to a hotmelt process. Following polymerization andblending with the photobase generator, the solvent was removed and thenthe hotmelt PSA was kneaded in a hotmelt extruder at 120° C. for 6hours. In none of Examples 1 to 6 was an increase in torque found duringthis period, which would have indicated gelling and hence thermalinstability. To confirm this, the gel index was determined as well. Thegel indices measured were all 0 and therefore additionally confirmed thethermal stability of the acrylate hotmelt PSAs.

In order to examine their suitability as PSAs the adhesive performanceproperties were likewise determined. In addition, a reference sample R1was included, which contains exactly the same comonomer composition asExample 1 but without any photobase generator. Since the adhesiveperformance properties in particular are dependent on the successfulcrosslinking of the hotmelt PSAs, the table below likewise lists the UVirradiation doses in the UV-C and UV-B range. TABLE 1 SWT 5 N, SWT 10 N,70° C. Ex- UV dose RT [min] [min] BS-steel [N/cm] ample [mJ/cm²] (testA1) (test A2) (test B) 1 UV-C: 150 +10000 +10000 4.1 UV-B: 840 R1 UV-C:150 325 15 1.8 UV-B: 840 2 UV-C: 150 +10000 +10000 4.2 UV-B: 840 3 UV-C:150 7685 4520 3.9 UV-B: 840 4 UV-C: 200 1240 2295 4.3 UV-B: 1120 5 UV-C:200 1550 1155 4.2 UV-B: 1120 6 UV-C: 300 2475 3420 5.1 UV-B: 168050 g/m² application rate to PET filmSWT: shear withstand timesBS: bond strength

A comparison of Examples 1 and R1 shows that as a result of adding thephotobase generator the crosslinking behavior of the PSA is markedlyimproved. The cohesion (shear withstand times) of Example 1 are muchhigher. Moreover, in the course of the bond strength measurement, thereference example splits cohesively, so that here it can be assumed thatthe PSA tape has undergone virtually no crosslinking. Examples 2 to 5establish the fact that different photobase generators can be used, thatthe cohesion of the PSA can be controlled ultimately by the comonomercomposition of the acrylate hotmelt PSAs of the invention, and thatdifferent functional groups can be used for crosslinking. Example 6affords the possibility of producing structured PSAs too by means oftargeted crosslinking via the PSAs of the invention.

1. A polyacrylate pressure-sensitive adhesive comprising a polymerformed from a) a comonomer mixture comprising a1) 55%-99% by weight,based on component (a), of acrylic acid and/or acrylic esters of theformula:CH₂═C(R¹)(COOR²),  where R¹═H or CH₃ and R² is an alkyl chain having1-20 carbon atoms, a2) 0-30% by weight, based on component (a), ofolefinically unsaturated monomers having functional, aromatic,heteroaromatic or heterocyclic groups, or a combination of any of saidgroups, wherein said functional groups are selected from the groupconsisting of hydroxyl groups, sulfonic acid groups, ester groups, ethergroups, anhydride groups, epoxy groups, amide groups, and amino groups,a3) 1%-15% by weight, based on component (a), of acrylate ormethacrylate monomers having at least one functional group, which iscapable of reacting with a photochemically generated base b), with orwithout addition of a catalyst, the polymer being thermally crosslinkedat least partly with 0.01%-25% by weight, based on the overall polymermixture, of a base b).
 2. The polyacrylate pressure-sensitive adhesiveof claim 1, wherein the monomers a1) are selected from the groupconsisting of acrylic monomers which comprise acrylic and methacrylicesters having alkyl groups consisting of 4 to 14 carbon atoms.
 3. Thepolyacrylate pressure-sensitive adhesive of claim 1, wherein themonomers a3) are selected from the group consisting of comonomerscontaining at least one carboxylic acid group, one isocyanato group orone epoxide group.
 4. The polyacrylate pressure-sensitive adhesive ofclaim 1, wherein said photochemically generated base is generated from aphotobase generator selected from the group consisting of O-acyl oximes,anilide derivatives, ammonium salts and organometallic compounds whichliberate a base under UV irradiation.
 5. A process for preparingpolyacrylate hotmelt pressure-sensitive adhesives from polymers formedfrom a) a comonomer mixture comprising a1) 55%-99% by weight, based oncomponent (a), of acrylic acid and/or acrylic esters of the formula:CH₂═C(R′)(COOR²),  where R¹═H or CH₃ and R² is an alkyl chain having1-20 carbon atoms, a2) 0-30% by weight, based on component (a), ofolefinically unsaturated monomers having functional groups selected fromthe group consisting of hydroxyl groups, sulfonic acid groups, estergroups, ether groups, anhydride groups, epoxy groups, amide groups andamino groups, or having aromatic, heteroaromatic and/or heterocyclicgroups, a3) 1%-15% by weight, based on component (a), of acrylate ormethacrylate monomers having at least one functional group which iscapable of reacting with the base generated by a photobase generator b),with or without a catalyzing compound, and b) 0.01%-25% by weight, basedon the overall comonomer mixture, of at least one photobase generator bwhere said photobase generator b) is incorporated into the comonomermixture by mixing or copolymerization and where the solvent-free polymeror the polymer substantially freed from solvent, with the photobasegenerator incorporated therein, is coated, in a hotmelt process, onto abacking, and during or after coating is irradiated with UV light,thereby generating a base photochemically, and the composition issubsequently crosslinked thermally by the reaction of at least componenta3) with the base.
 6. The process of claim 5, wherein solvent, ifpresent, is removed with heating under reduced pressure.
 7. The processof claim 5, wherein the polymer is placed onto a film of water, withsubsequent transfer from the film of water to the backing material, thewater optionally contributing to the crosslinking of thepressure-sensitive adhesive.
 8. The process of claim 5, wherein said UVirradiation takes place during coating.
 9. The process of claim 5,wherein the polyacrylate pressure-sensitive adhesive on the backingmaterial is irradiated with UV light over its full area and subsequentlyheated, for the purpose of thermal crosslinking, to a temperature of atleast 80° C.
 10. The process of claim 5, wherein structuredpolyacrylates are prepared by performing a structured crosslinking byirradiating the polymer coating with ultraviolet light in such a waythat only certain regions of the polymer mixture are exposed to the UVradiation.
 11. The process of claim 10, wherein the polymer coating isirradiated with ultraviolet light through a perforated mask.
 12. Theprocess of claim 10, wherein the polymer coating is irradiated withultraviolet light through a film whose surface has regions of differentUV light transparency, whereby certain regions of the polymer mixtureare exposed to different intensities of UV radiation. 13.Pressure-sensitive adhesive tapes and strips coated on one or both sideswith the polyacrylate pressure-sensitive adhesive of claim
 1. 14. Thepressure-sensitive adhesive of claim 2, wherein said alkyl groupsconsist of 4-9 carbon atoms.
 15. The pressure sensitive adhesive ofclaim 14, wherein said monomers a1) are selected from the groupconsisting of n-butyl acrylate, n-pentyl acrylate, n-hexyl acrylate,n-heptyl acrylate, n-octyl acrylate, n-nonyl acrylate, lauryl acrylate,stearyl acrylate, behenyl acrylate, and their branched isomers.
 16. Thepolyacrylate pressure-sensitive adhesive of claim 3, wherein themonomers a3) are selected from the group consisting of glycidylmethacrylate, acrylic acid, methacrylic acid and 2-isocyanatoethylmethacrylate.
 17. The process of claim 9, wherein said temperature is upto about 100° C.